GaAs-HBT (Heterojunction Bipolar Transistor) power amplifiers have been widely used as cellular phone power amplifiers for CDMA, etc. and as wireless LAN power amplifiers. Unlike GaAs-FET power amplifiers, GaAs-HBT power amplifiers do not require a negative gate bias voltage, which allows them to operate on a single power source. Furthermore, their device characteristics do not vary as much as those of GaAs-FET power amplifiers.
FIGS. 23, 24, and 25 are circuit diagrams of GaAs-HBT power amplifiers. In these figures, symbol IN denotes an input terminal; OUT, an output terminal; Tr11, Tr12, and Tr13, GaAs-HBTs; Cc1, Cc2, and Cc3, capacitances; Vc1, Vc2, and Vc3, power supply voltages; R0, R1, R2, and R3, resistors; IM1, IS1, IS2, OM1, matching circuits; and Coup1, a directional coupler. The wave detector circuit for monitoring the output power of such a power amplifier is mounted on the board of the terminal device or in the power amplifier module (see, e.g., Japanese Laid-Open Patent Publication Nos. 2-80972 and 2005-109644 and International Patent Application Publication No. WO 99/37019).
The wave detector circuits shown in FIGS. 23 and 24 directly monitor the RF power at the collector and the base, respectively, of the final stage transistor Tr13. The wave detector circuit shown in FIG. 25, on the other hand, monitors the RF power through the coupler Coup1, which allows accurate monitoring of the power of the forward propagating wave.
The power monitoring circuit shown in FIG. 23 is simple and may be used when the accuracy requirements are not so stringent, since the power monitored by the wave detector circuit includes reflected wave power when a load change occurs. For example, it is relatively common to use this power monitoring circuit in a LAN terminal, which is less mobile than a cellular phone. On the other hand, the power monitoring circuit shown in FIG. 24 does not monitor the output power from the final stage, since the wave detector circuit is provided on the base side of the final stage transistor Tr13, as described above, resulting in slightly inaccurate power monitoring. However, there is no problem with using this power monitoring circuit when monitoring a modulated signal in OFDM (Orthogonal Frequency Division Multiplexing), which has been used in wireless LAN systems and WiMAX systems in recent years. The reason for this is as follows. The average operating output power of OFDM power amplifiers is approximately 8-10 dB lower than their saturated output power; that is, they operate in a perfectly linear range. Therefore, the output power of the transistor Tr13 in FIG. 24 depends only on its input power and gain, which is linear. Further, since the base-collector junction of the transistor Tr13 provides isolation and the wave detector circuit is connected to the input side (i.e., the base) of the transistor Tr13, the wave detector circuit can accurately monitor the power of the forward propagating wave without using a coupler even when a load change occurs.
FIGS. 26 and 27 are circuit diagrams of conventional RF wave detector circuits. In these figures, symbol Vin denotes an RF input voltage; Vref, a reference DC voltage; Vdet, a detected voltage (or output voltage); Rin, Rref1, Rref2, Ra1, Ra2, and Ra3, resistors; Rout, a smoothing resistor; Tr1 and Tr2, HBTs; Cin, a DC blocking capacitance; Cout, a smoothing capacitance; and OP1, an operational amplifier. The detected voltage (or output voltage) Vdet is usually proportional to the RF input voltage, and the input power Pin is proportional to the square of the RF input voltage. As a result, the input power Pin is proportional to the square of the detected voltage Vdet, as shown in FIG. 28.
Since an HBT usually has very low input impedance, the voltage at its base has a much lower amplitude than the voltage at its collector. Therefore, if the simple diode detector circuit shown in FIG. 26 is connected to the base of the final stage transistor Tr13 in the power amplifier shown in FIG. 24, the detector circuit may not able to detect the RF signal since the amplitude of the voltage at the base of the transistor Tr13 is very low. The differential diode detector circuit shown in FIG. 27, on the other hand, can detect small variations in voltage. Further, the detector circuit shown in FIG. 27 exhibits reduced variation in detected voltage with temperature, as compared to the detector circuit shown in FIG. 26.
However, while all components of the detector circuit shown in FIG. 26 can be manufactured by a GaAs-HBT process, the operational amplifier OP1 of the detector circuit shown in FIG. 27 cannot be formed by a GaAs-HBT process. Therefore, in the case of the detector circuit shown in FIG. 27, an Si circuit (namely, the operational amplifier OP1) must be mounted on the board or the module substrate. That is, the detector circuit shown in FIG. 27 is disadvantageous in that it cannot be integrally formed with the power amplifier on the GaAs-HBT substrate.